Does walking 3 miles a day count as cardio?

It depends on the intensity. You should be at (or work up to) at least a 15 minute mile pace. That's 3 miles in 45 minutes. Walk heel to toe, and keep arms at a 45 degree angle, pump back and forth (but not across the body.) If you start to feel tired, pump your arms more, your legs will have to walk faster to keep up! (Just a little trick I've learned.)